Caster wheel with parabolic tread-hub interface

ABSTRACT

A caster wheel assembly includes at least one hub that has an annularly dished concave region disposed between a pair of cylindrical lips about its outer periphery. An elastomeric tread has a convex inner periphery that seats in the concave region of the hub. The interface between the hub and tread is parabolic in cross-section to manage load-inducted stresses in the tread. The parabolic cross-section is defined by the equation y=A*x 2 , where A is between 1.5 and 4. A cylindrical lip/pad interface is established between hub and tread on opposite sides of the parabolic interface to accommodate high load situations. A bearing is supported in the hub, and a bushing inside the bearing. An axle shaft passing through the bushing attaches the hub and tread to a support bracket for use in a wide variety of industrial applications.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/103,126, filed Dec. 11, 2013, which claims priority to ProvisionalPatent Application No. 61/735,559 filed Dec. 11, 2012, the entiredisclosures of which are hereby incorporated by reference and reliedupon.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to casters having one or more wheels ofspecific construction, and more particularly to a caster wheel having aresilient tread with engineered rim or hub interface to more effectivelyaccommodate a wide range of loading stresses.

2. Description of Related Art

A caster wheel is single, double, or compound wheel that is designed tobe mounted to the bottom of a larger object, or vehicle, so as to enablethat object to be easily moved. The term “caster wheel” as used hereinis intended to apply to both driven and un-driven configurations,although un-driven or free-wheeling configurations are more common. Highcapacity, heavy duty casters are used in many industrial applications,such as platform trucks, carts, assemblies, and tow lines in plants.

A standard caster wheel has a center rotating hub (with or without abushing or bearing) and a compliant tread material applied about itsouter periphery as a rolling contact surface. The outer diameter of acaster wheel affects how easy it is for the caster to be able to moveacross rough or irregular surfaces. Large diameter caster wheels areable to bridge wide gaps, such as between an elevator door and anelevator car. In situations where heavy loads need to be transported oncasters, load capacity may be increased by using wider wheels with moreground contact area. However, when rotating in-place a wide swivelcaster, the center part of the wheel-to-ground contact patch rotatesslower than the regions further out to the sides. This difference inrotation speed across the base of the wheel contact patch causes widewheels to resist rotation around the point of swivel, a resistance whichincreases as weight loading increases.

An alternate way to increase load capacity while limitingswivel-rotation resistance is to use multiple narrow wheels in tandem onthe same wheel axis, as shown for example in FIG. 1. Each wheel of adual-wheel swiveling caster has a comparatively narrower ground contactpatch (i.e., footprint) than a single wide wheel and each wheel is ableto rotate independently at a different rate, so there is less resistanceto turning in place on the swivel. There are several scenarios thatdictate the use of a dual-wheel caster in an industrial application.

A first one of these scenarios is when the anticipated load cannoteasily be carried by one wheel. When the anticipated load to betransported exceeds the load rating of one wheel then a dual-wheelconfiguration can be an excellent solution. A second scenario occurswhen there is a need to reduce the height of the cart or platform. Oneway to reduce the height of a cart or platform that is fitted forcasters is to spread the load over more wheels. When the load is spreadover twice (2×) the number of wheels, the diameter of the wheel can bereduced as the calculated load on each wheel is halved. The use of adual-wheel caster will generally reduce the overall height byapproximately 30% while carrying the same load. Thirdly, dual-wheelcasters are necessary when there is the need to reduce per square inchof floor loading. Commonly in applications where there is restriction onper square inch loading of floors (due to structural concerns or surfaceconcerns) the load can be proportionally spread across a greater wheelsurface. Still further, dual-wheel casters enable easier swiveling whena cart is fully loaded. Generally stated, the heavier the load is oneach wheel (greater load per square inch) the more force it will take toswivel a caster assembly. The use of dual-wheels halves the per-wheelload and thus can allow assembly swiveling to take place with lessforce. As a dual-wheel caster assembly swivels, one wheel will oftenturn clockwise while the other wheel rotates counter clockwise. Ineffect, the vertical swivel axis is located between the two wheels,thereby reducing wheel scrubbing and making the swiveling of heavy loadssimpler.

Certain uses may require a caster wheel to have a resilient treadmaterial around the outer perimeter of the wheel. The tread can be madeof many elastomeric materials and take on different shapes, such as apneumatic tire, a coating of polyurethane or over-molded elastomer. Thehardness of a caster wheel tread affects ease of rolling, traction,durability, shock absorption, and noise. Hard wheels are easier to rollon smooth surfaces, but are noisier and provide less traction. Softwheels are easier to push on rough surfaces, quieter, protect the floor,absorb shock and provide better traction, but have a lower loadcapacity. This presents a problem in situations where a heavy load needsto be transported with the positive effects of soft wheels, for example,good traction, shock absorption and floor protection. If an elastomerictread is omitted altogether to increase load capacity, then the positiveaspects of the tread are also removed.

The radial thickness of the tread will affect the rollingcharacteristics of a caster wheel. All things being equal, thin treadsare easier to push while thick treads have better shock absorption.Again, in industrial situations where ease of transportation by pushingcombined with good shock absorption is key, the design engineer isfrequently left with a compromise or trade-off between low rollingresistance and good shock absorption.

Furthermore, the working life of an elastomeric tread tends to definethe working life of a caster wheel assembly. That is, the tread tends tobe the primary wear part of a caster wheel assembly. When the tread lifeis near its end, the entire caster assembly is typically replaced or atleast the worn wheel is replaced. One factor contributing to acceleratedtread wear is shear stresses propagating through the elastomeric treadmaterial. Elastomeric materials of the type used for caster wheel treadstend to be strong and durable under compression, but substantially morefragile in shear. Working life is also diminished by excessive heatbuild-up in the tread. Heat is generated while the caster wheel isrolling and the tread is being deformed. Tread thickness has an effecton heat dissipation; thicker treads tend to dissipate heat more slowlyinto the hub (a heat sink), and thus accelerate wear.

There is a need for a caster wheel that behaves like a thin/hard treadwheel, but has the good characteristics of a thick/soft tread. Morestill, such a wheel should be readily adaptable to multi-wheelapplications that permit a heavy load to be transported on a soft andthin tread material. And even more specifically, there is a need for anoptimized tread design with improved load carry capabilities with theability to better distribute load stresses so as to remove or at leastreduce shear effects. Still further, there is a need for a tread designthat facilitates heat removal into the hub.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of this invention, a caster wheel assemblyis provided comprising a hub that has a generally annular body centeredabout a central axis. The hub has an outer peripheral rim section and aninner bearing section. The rim section includes an annular outer rimsurface with an annularly dished concave region disposed between a pairof cylindrical lips. A tread surrounds the rim surface. The tread has aconvex inner periphery conforming to the annularly dished concave regionof the rim surface in a tight nested relationship. The tread alsoincludes a cylindrical pad on each side of the convex inner periphery.The cylindrical pads are disposed in direct opposing relation to thecylindrical lips of the rim surface. The annularly dished concave regionof the outer rim surface has a parabolic OD cross-section as takenthrough a plane extending radially from the central axis.

The parabolic OD cross-section of the outer rim surface facilitates animproved distribution of load-induced stresses within the tread towardcompression mode. In other words, the parabolic OD cross-section of theouter rim surface helps re-orient load stresses within the tread awayfrom shear mode. This stress re-positioning (from shear towardcompression) allows the tread to carry more load and/or enjoy a longerworking life than a wheel of prior art design. The resulting stressesare also more uniform throughout the tread thereby reducing the risk ofcreating adverse stress concentrations which can lead to prematurefailure of the tread. Furthermore, the parabolic OD cross-section of theouter rim surface improves heat dissipation from the tread into the hub.

According to another aspect of this invention, a multi-wheel casterwheel assembly comprises at least two hubs. Each hub has a generallyannular body centered about a common central axis, and each hub has anouter peripheral rim section and an inner bearing section. Each rimsection includes an annular outer rim surface having an annularly dishedconcave region disposed between a pair of cylindrical lips. At least twotreads are provided, each one surrounding a respective one of the rimsurfaces. Each tread has a convex inner periphery conforming to theannularly dished concave region of the rim surface in a tight nestedrelationship. Each tread also includes a cylindrical pad on each side ofthe convex inner periphery. The cylindrical pads are disposed in directopposing relation to the cylindrical lips of a respective one of the rimsurfaces. The annularly dished concave region of each outer rim surfacehas a parabolic OD cross-section.

The parabolic OD cross-sections of each outer rim surface provide adistinct load carrying advantage. Stresses emanating from the groundcontact point will radiate outward more evenly across the entire surfaceof the tread as compared with prior art (i.e., non-parabolic) designs.The parabolic interface between hub and tread helps re-orient the linesof stress within the elastomeric tread material toward compressionthereby increasing the chances that the tread material will endure shearstresses. This arrangement thus optimizes the load carry capabilities ofthe caster wheel assembly by improving distribution of the load stressesalong the entire surface of the tread and reduces adverse shear effectsto thereby increase working life and/or load-carrying capacity and alsoreduces heat build-up in use.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

These and other features and advantages of the present invention willbecome more readily appreciated when considered in connection with thefollowing detailed description and appended drawings, wherein:

FIG. 1 is perspective view of a prior art dual-wheel swiveling caserassembly;

FIG. 2 is front view of a caster wheel assembly according to oneembodiment of the present invention, with a portion of the supportingbracket shown in phantom;

FIG. 3 is a cross-section taken general along lines 3-3 of FIG. 2;

FIG. 4 is a perspective view of the hub and tread;

FIG. 5 is an exploded view of the hub and tread as depicted in FIG. 4;

FIG. 6 is a fragmentary view showing the tread in cross-section as takengenerally along lines 6-6 of FIG. 4;

FIG. 7 is an enlarged view of the area indicate at 7 in FIG. 3;

FIG. 8 is an enlarged view of the area indicate at 8 in FIG. 3;

FIG. 9 is a view as in FIG. 8 but showing the focal and vertex points ofthe parabolic interface between hub and tread; and

FIGS. 10A-10D are enlarged views showing the point of contact betweentread and ground through progressively increasing loads, with thecorresponding distribution stresses through the elastomeric tread bodyremaining substantially in compression mode.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 2-10D, wherein like numerals indicate like orcorresponding parts throughout the several views, a caster wheelassembly is generally shown at 20. The assembly 20 is of the type usedin either a multi-wheel assembly or in a single wheel assembly and maybe power-driven or un-driven Likewise, the assembly 20 may be configuredas a swiveling type unit, a steerable type unit, or a non-swiveling(fixed) type. For exemplary purposes, the caster wheel assembly 20 isshown in an optional dual-wheel configuration in FIGS. 3 and 7.

The assembly 20 includes a hub, generally indicated at 22. In dual-wheelconfigurations like that of FIGS. 3 and 7, two generally identical hubs22 are provided. Three and more-wheel configurations as likewisepossible. For convenience, much of the following descriptions willreference the hub 22 in the singular as if in a single-wheel embodiment.However, it should be understood that multi-wheel configurations willinclude multiple such hubs 22 of identical or nearly identical form. Thehub 22 comprises a generally annular body centered about a horizontalcentral axis A. The central axis A is the rolling axis of the assembly20. As perhaps best shown in FIGS. 3 and 5, the hub 22 has an outerperipheral rim section 24 and an inner bearing section 26. The bearingsection 26 is closest to the central axis A, whereas the rim section 24is most distal. An annular web section 28 fills the region between therim section 24 and the bearing section 26. The hub 22 is preferably, butnot by necessity, a monolithic structure and may be formed from any ofvarious suitable materials including metal, plastic or compositesdepending on expected loads and usage. In a preferred embodiment, thehub 22 is made from an aluminum die-cast core for maximum strength.Although the hub 22 is shown as a simple solid structure, the specificdesign may take many forms including spoke designs, particularly in theweb section 28, integrated suspension features, braking features, andthe like. In short, the hub 22 shown here is intended to represent anytype or design of hub used in any type of caster wheel application.

In FIG. 3, the hub 22 is shown with its rim section 24 and bearingsection 26 having a generally equal axial width. That is, the width ofthese sections 24, 26 as measured axially, is generally the same. Thespecific widths of these sections 24, 26 will vary from one applicationto the next, and it is foreseeable that in some applications the rimsection 24 may be narrower or possibly even wider than the width of thebearing section 26. Also evident from FIG. 3, the web section 28 mayhave an axial width that at most or all points is less than the axialwidths of the rim section 24 and the bearing section 26. In theillustrated example, the web section 28 has a relatively narrow axialwidth that transitions gracefully into each of the rim 24 and bearing 26sections. As mentioned earlier, the web section 28 can be designed inany number of configurations depending on the application and materialused. The web section 28 may also be separately fabricated andsubsequently assembled to the rim 24 and bearing 26 sections.

In FIG. 7, the bearing section 26 of the hub 22 is shown including aninner bearing seat defined by a counter-bore 30 and a stop ridge 32.Both the counter-bore 30 and the flange-like stop ridge 32 are centeredabout the central axis A. In dual-wheel embodiments, it may beadvantageous to orient the stop ridge 32 of each hub toward the inside.That is, as depicted in FIGS. 3 and 7, the counter-bores 30 indual-wheel applications may open toward the outside of each hub 22. Aring groove 34 may be fashioned in the counter-bore 30 to receive a snapring 36, as will be described below.

Turning now to the rim section 24, reference is made to FIGS. 5, 6, 8and 9. As shown in these views, the rim section 24 includes an annularouter rim surface which is characterized by an annularly dished concaveregion 38 disposed between a pair of cylindrical lips 40. That is tosay, the outer perimeter of each hub 22 is annularly dished at theconcave region 38. The concave region 38 is centrally located, andflanked on each side by a thin flat (i.e., flat when viewed incross-section) lip 40.

The annularly dished concave region 38 of the outer rim surface has aparabolic OD cross-section as taken through a plane extending radiallyfrom the central axis A. The parabolic OD cross-section is perhaps bestshown by the broken line in FIG. 9 including an OD focal point B and anOD vertex C. An axis of symmetry D passes radially through the OD focalpoint B and the OD vertex C, and intersects the central axis A. Forpurposes of caster wheel applications it has been found that a parabolicshape defined by the equation y=A*x² will produce a suitable curve forthe concave region 38 (as viewed through any cross-section takenradially through the central axis A). In this equation, the letter “A”is a constant which is ideally set within the range of about 1.5-4. Morespecifically, a parabolic constant “A” of about 3.6 has been found toprovide superior results when the concave region 38 has a depth of about0.5 inches and a width of about 0.75 inches. Even more specifically, aparabolic constant “A” of 3.5556 will proved superb results. Thus, theparabolic curvature in this specific example can be expressed asy=3.5556*x², or to be less specific y≈3.6*x², or less specific stilly≈(1.5≦A≦4)*x².

Preferably, but not necessarily, the OD focal point B is generallylocated between the OD vertex C and the cylindrical lips 40 at anyparticular radial cross-section. By “generally located” it is meant thatthe OD focal point B is preferably within this range but could belocated further from the OD vertex C in some cases. In the illustratedexamples of FIGS. 9 and 10A-10B, the OD focal point B is set relativelyclose to the OD vertex C. In the specific example above where theparabolic curvature is expressed as y=3.5556*x², the OD focal point maybe approximately 0.07″ away from the OD vertex C. Of course, the ODfocal point B could be set more distal from OD vertex C than the lips40, and yet still provide acceptable results. For reasons that will bedescribed in greater detail below, enhanced operating characteristics ofthe assembly 20 can be realized when the OD focal point B is located inany of a wide range of positions along the axis of symmetry D.

In the disclosed embodiment a bearing, generally indicated at 42, isdisposed inside the counter-bore 30 of the hub 22, abutting the stopridge 32. As perhaps best shown in FIG. 7, the bearing 42 may comprise aroller bearing having a plurality of roller elements 44 trapped betweeninner 46 and outer 48 races. The annular outer race 48 may bedimensioned for a press-fit or interference fit in the counter-bore 30to assure that the outer race 48 is seated in and rotates with the hub22 in use. The bearing 42 is secured in the counter-bore 30 with thesnap ring 36. The snap ring 36 is seated under spring tension in thering groove 34 and bears against the outer race 48 so as to contain thebearing 42 in an operative position in the counter-bore 30 even underthe influence of strong side-loading. Although a roller type bearing 42is preferred for many applications, it should be understood that slidingbearings as well as simple bushings could be used under appropriatecircumstances. Each wheel can therefore be fitted with a variety ofdifferent bearing types. The bearing type should be a consideration withdesigning a wheel into a specific application. For example, rollerbearings are often selected for manual applications and walking speeds,precision ball bearings are typically selected for higher speedapplications that take less abuse or that may need to be sealed formaintenance free applications, and tapered bearings are used for themost abusive applications and heaviest loads.

An axle system interacts with the bearing 42 along the central axis A.The axle system includes a bushing, generally indicated at 50, which hasa cylindrical outer surface 52 disposed inside the circular opening ofthe inner race 46. The annular opening inside the inner race 46 may bedimensioned for a press-fit or at least relatively tight fit over thebushing 50 so that the inner race 46 and rotates with the bushing 50 inuse. The bushing 50 may include a flange 54 configured to abut an axialface of the bearing inner race 46. In the illustrated examples, thebushing 50 is provided with a central through-hole 56 extending alongthe central axis A. An axle shaft 58 is disposed in the through-hole 56.In contemplated alternative embodiments, the bushing 50 may be formedwith integrated pintles or journals that obviate the need for a separateaxle shaft 58. Many other options exist.

In dual-wheel embodiments, the bushing 50 may comprise interlockinghalf-sections, with each half-section associated with a different one ofthe bearings 42 and holding both in axial alignment. As shown in FIGS. 3and 7, the interlocking half-sections of the bushing 50 may matetogether at a male-female interface 60. Naturally three-wheel (and more)embodiments are possible by simple duplication of features. Inmulti-wheel applications, the bearings 42 may be separated from oneanother by an annular spacer 62. The spacer 62 is shown best in FIG. 7.The inner diameter of the spacer 62 is preferably equal to the innerdiameters of the bearings 42 so that they are all centrally supported onthe one common bushing 50. That is to say, in multi-wheel applications aspacer 62 is located each two adjacent bearings 42 to maintain apreferred separation between the stacked wheel assemblies. The outerflanges 54 of the bushing 50 butt against the inner races 46 of the twooutside bearings 42 to sandwich all of the inner bearing races 46 andspacer(s) 62 into a generally unified, connected, compressed structure.An axle shaft 58 slides through a common through-hole 56 in the bushing50 to support the multiple hubs 22 for independent rotation.

As shown in FIGS. 2 and 3, a support bracket 63 interacts with the axleshaft 60 to attach the assembly 20 to a cart or other vehicle in use.The support bracket 63 is shown in this illustrative example including apair of legs positioned on either side of the hub 22. Each leg includesan axle hole aligned with the bushing through-hole 56 to receive theaxle shaft 60 and thus support the hub 22. The support bracket 63 may ofcourse take many forms within the spirit of this invention, and mayinclude swiveling, braking and other common features as needed.

A tread, generally indicated at 64, surrounds the outer rim surface ofthe rim section 24. The tread 64 is preferably fabricated from aresilient elastomeric material, such as polyurethane or other suitablerubber or rubber-like compounds. Of course, the tread 64 may be formedfrom a variety of other resilient materials. In practice, the tread 64can be over-molded onto the hub 22, or loose-piece molded andsubsequently assembled to the hub 22 (as alluded to in the exploded viewof FIG. 5). The tread 64 seats in a snug nested relationship within thevarious contours of the outer rim surface of the hub 22. In particular,the tread 64 is shown having a convex inner periphery 66 conforming tothe annularly dished concave region 38 of the rim surface in a matedrelationship. The tread 64 also includes a cylindrical pad 66 on eachside of the convex inner periphery 66. That is to say, a pair ofcylindrically-shaped pads 68 are formed on each side of the convex innerperiphery 66 and in direct opposing relation to the cylindrical lips 40of the rim surface. The tread 64 has an outermost rolling surface thatis convexly rounded and bounded on either side by flat sidewalls 70. Thewidth of the sidewalls 70 is preferably equal or generally equal to thewidth of the rim section 24.

Referring again to FIGS. 8 and 9, the convex inner periphery 66 of thetread 64 also has a parabolic ID cross-section as taken through a planeextending radially from the central axis A, like its mirrored concaveregion 38. That is, the tread 64 completely fills the rim outerperiphery so that there are no voids or pockets of air therebetween.Preferably, the extrapolated pattern of the parabola at any givencross-section intersects the exposed surface of the tread 64 at or nearthe point of transition between the flat sidewalls 70 and the convexrolling surface of the tread 64. As shown in FIG. 9, in one exemplaryembodiment the extrapolated pattern of the parabola intersects theconvex rolling surface of the tread 64 at or just inside the sidewall 70transition points.

The parabolic ID cross-section has an ID focal point that coincides withthe OD focal point B. The ID focal point, like the OD focal point B, cantherefore be located in any of a wide range of positions along the axisof symmetry D. In the specific example above where the paraboliccurvature is expressed as y=3.5556*x², the ID focal point may be locatedapproximately 0.07″ away from the OD vertex C to provide suitableperformance. This and other positions of the ID focal point areconsidered within acceptable operating parameters.

An ID vertex coincides with the OD vertex C, and the axis of symmetry Dis shared by the convex inner periphery 66 of the tread 64 and theconcave region 38 of the rim section 24. As mentioned several times, theprecise location of the parabolic focal point is intended to begenerously defined and to include locations of the ID focal point bothless as well as more distal from the ID vertex. In terms of a range, thecoincident ID/OD focal points B may be located almost anywhere withinthe body of the tread 64. That is, the ID/OD focal points B should liealong the axis of symmetry D somewhere between the convex crest of thetread's rolling surface and the OD vertex C. Various advantages can bederived by strategically locating the parabolic focal points not only inthe above-describe example but also so as to lie somewhere between theOD vertex C and the axial alignment of the lips 40/pads 68.

Referring now to FIGS. 10A through 10D, the tread 64 is shown respondingto progressively increasing loads. Under light loads (FIG. 10A), theground contact patch (i.e., footprint) of the assembly 20 is relativelysmall and transmits forces through the thickest portion of the tread 64in a conically spreading manner into the concave region 38. The effectis substantial tread 64 resiliency and shock absorption, as the meatiestpart of the tread 64 is taking substantially all of the stressesgenerated by the weight forces. As progressively greater loads arecarried through the assembly 20, the reaction forces spreadprogressively outwardly, somewhat truncating the conical stressdistribution. For example, FIG. 10B represents somewhat greater loadingas compared with the respective preceding illustrations of FIG. 10A.With greater loading, the ground contact patch widens, thus transmittingforces and stresses through a widening portion of the tread 64 but stillcentered about the axis of symmetry D. FIG. 10C represents still greaterloading as compared with the respective preceding illustrations of FIG.10B. As shown, the ground contact patch widens still further,transmitting forces and stresses through an even wider portion of thetread 64. FIG. 10D depicts yet still higher loading conditions in whichall or substantially the entire convexly rounded outermost portion ofthe tread 64 has been flattened under load. At this loading level andabove, a significant portion of the force distribution is finallytransmitted through the lips 40.

As can be appreciated by one of skill in the art, under light loadingthe assembly 20 will behave like a traditional thick tread caster wheelwith good shock absorption, but under very heavy load the assembly 20will behave like a prior art thin tread design that is easy to roll. Theunique interface between hub 22 and tread 64 enables the present casterassembly 20 to behave like a traditional thin tread wheel under heavyloads, while retaining also the traditional good characteristics of athick tread when the loading is light. The assembly 20 is readilyadaptable to multi-wheel applications that permit heavy loads to betransported on a soft and thin tread material. The assembly 20 will thushave a very long tread life and offer substantially maintenance freeservice.

The particular benefits of the parabolic tread inner periphery 66/rimsection 24 interface will be described in further detail. Most materialsare stronger under compression than under a shear or tensile load.Elastomers, from which the tread 64 is composed, are materials that canbe expected to behave optimally in compression. The convex shape of theouter rim section 24, paired with complimentary inner periphery 66 ofthe tread 64, causes the predominant resulting load-induced stresseswithin the tread 64 to orient in compression. By manipulating the shapeof the propagating stress through the back-stopping parabolic interface,the assembly 20 of this invention is able to carry more load than acomparable prior art wheel without a parabolic interface and/or to carryhigh loads with less heat build-up. The novel parabolic interface ofthis invention causes the load-induced stresses to be more uniformthroughout the tread 64 thereby reducing the risk of creating localizedstress concentrations leading to failure of the tread 22. Furthermore,heat is generated by the tread 64 when deformed under load andconcurrently rolled. Greater loading and/or rolling speed causes more(or more frequent) deformation, which in turn causes greater heatbuild-up in use. The parabolic interface enables the tread 64 to achievethe favorable characteristics of a thick tread (e.g., shock absorption)but without the problematic heat build-up issues of a prior art thicktread wheel by managing the shape of the load-induced stresses withinthe tread 64. In particular, the parabolic interface reduces the sheareffects that contribute to heat build-up, and also enables heat thatdoes inevitably build-up within the body of the tread 64 (due to rollingdeformation) to be more directly removed into the hub 22.

As depicted in FIGS. 10A-10D, the parabolic tread inner periphery 66/rimsection 24 interface provides distinct load carrying advantages ascompared with non-parabolic designs of the prior art. Stressesoriginating at the ground contact point will radiate evenly across theentire body of the tread 64. Ground contact can be assumed to occuralong a relatively flat surface, i.e., the ground. While it may beappreciated by those of skill in the art that load-induced stresses donot reflect from a parabolic surface as sound or light rays would, FIGS.10A-10D are nevertheless instructive to illustrate how stress linesemanating from a flat ground contact patch will encounter a parabolicinterface and orient more or less in compression to make effective useof the resilient tread material with minimal shear stresses. A pointload applied to the outer tread surface will produce stress in aconically propagating shape from the force in the material beingdeformed. As a result, the stress in the tread 64 emanating from theflat ground contact patch will spread through the body of elastomericmaterial. The goal of the convex shape formed at the tread innerperiphery 66/rim section 24 interface of this invention is to resistmost or all of the conically propagating stress on the concave innerregion 38 of the rim 24 against an opposing surface that will maintainthe stress within the body of the tread 64 under compression or as closeto compression as possible. A parabolic interface has been found toprovide an ideal configuration for maintaining the load-induced stresseswithin the elastomeric tread 64 under compression, when it is consideredthat the opposing ground contact patch is generally flat/planar. Thatis, the parabolic interface has been found to be the ideal configurationas a surface shape in opposition to the flat ground patch whereload-induced stresses are formed therebetween, so that shear stressesare minimized while compressive stresses are maximized.

When an elastomer material is deformed, it behaves somewhat analogous toan incompressible fluid and tends to bulge out on the sides. In a casterwheel application, where the tread is made from an elastomer, thatdeformation-induced overhang creates a shear zone within the elastomerthat as a result creates a material weak point. The novel parabolicinterface minimizes formation of the bulge negative effects so that thetread elastomer is placed predominantly throughout in compression. Aparabolic interface is advantageous over a rectangular or squareinterface profile because the unique qualities of a parabola provide afar more gradual transition in the stress propagations from a flatground patch. On the other hand, a parabolic interface is advantageousover a circular interface profile because, in a tread having comparabletread thickness, the sides of a corresponding circle shape would toorapidly turn vertical which would not adequately support the load incompression. By suitable thickness, it will be understood thatapplications will differ and dictate different proportions of tread/hubdimensions. Generally stated, however, for a tread that is too shallow,the elastomer stress cone produced under average loading is likely to gobeyond the concave interface boundary and thus create areas ofundesirable shear along the lateral regions of the tread. Accordingly,for caster wheel applications, the parabola can yield a far moregradually transitioning curve than any other geometric shape for thetread inner periphery 66/rim section 24 interface.

The unique tread 64 configuration of this invention optimizes the loadcarry capabilities of the assembly 20 by optimally distributing the loadalong the entire surface of the tread 64 and reducing adverse sheareffects. This configuration also minimizes heat build-up, providesimproved impact resistance, and maximizes the service life of theassembly 20. The design is particularly conducive to multi-wheelembodiments which further benefit from reduced friction withoutscrubbing or sliding when turning. The assembly 20 is readily adaptableto a wide variety of industrial applications, and provides easiermovement of loaded carts or vehicles. Caster wheel assemblies 20incorporating elements of this invention are excellent for productionracks (power transported, manually handled and mobile fixtures), partsbins, conveyor systems, and other applications where maintenance freecasters and ergonomics are desired.

The foregoing invention has been described in accordance with therelevant legal standards, thus the description is exemplary rather thanlimiting in nature. Variations and modifications to the disclosedembodiment may become apparent to those skilled in the art and fallwithin the scope of the invention.

What is claimed is:
 1. A multi-wheel caster wheel assembly comprising:at least two hubs, each said hub comprising a generally annular bodycentered about a common central axis, each said hub having an outerperipheral rim section and an inner bearing section, each said rimsection including an annular outer rim surface having an annularlydished concave region disposed between a pair of cylindrical lips, atleast two treads, each said tread surrounding a respective one of saidrim surfaces, each said tread having a convex inner periphery conformingto said annularly dished concave region of said rim surface in a tightnested relationship, each said tread including a cylindrical pad on eachside of said convex inner periphery, said cylindrical pads disposed indirect surface-to-surface contact with said cylindrical lips of arespective one of said rim surfaces, wherein said annularly dishedconcave region of each said outer rim surface has a parabolic ODcross-section, and wherein said parabolic OD cross-section is defined bythe equation y=A*x2, where A is between 1.5 and
 4. 2. The assembly ofclaim 1 wherein said parabolic OD cross-section is defined by theequation y=A*x², where A is about 3.6.
 3. The assembly of claim 1wherein each said tread is fabricated from a resilient elastomericmaterial.
 4. The assembly of claim 3 wherein each said tread includes anoutermost rolling surface, said outermost rolling surface being convexlyrounded.
 5. The assembly of claim 1 further including a bearing disposedinside said inner bearing section of each said hub, and a bushing havinga cylindrical outer surface disposed inside each said bearing.
 6. Theassembly of claim 5 further including an annular spacer disposed betweeneach of said bearings and about said cylindrical outer surface of saidbushing.
 7. The assembly of claim 6 wherein said bushing comprises asplit bushing having interlocking male and female top hat sections, saidmale top hat section operatively associated with one of said bearingsand said female top hat section operatively associated with the other ofsaid bearings.
 8. A caster wheel assembly comprising: a hub comprising agenerally annular body centered about a central axis, said hub having anouter peripheral rim section and an inner bearing section, said rimsection including an annular outer rim surface having an annularlydished concave region disposed between a pair of cylindrical lips, atread surrounding said rim surface, said tread having a convex innerperiphery conforming to said annularly dished concave region of said rimsurface in a tight nested relationship, said tread including acylindrical pad on each side of said convex inner periphery, saidcylindrical pads disposed in direct surface-to-surface contact with saidcylindrical lips of said rim surface, wherein said annularly dishedconcave region of said outer rim surface has a parabolic ODcross-section, and wherein said parabolic OD cross-section is defined bythe equation y=A*x², where A is between 1.5 and
 4. 9. The assembly ofclaim 8 wherein said parabolic OD cross-section is defined by theequation y=A*x², where A is about 3.6.
 10. The assembly of claim 8wherein said tread is fabricated from a resilient elastomeric material.11. The assembly of claim 10 wherein said tread includes an outermostrolling surface, said outermost rolling surface being convexly rounded.12. The assembly of claim 8 wherein said rim section and said bearingsection have generally equal axial widths.
 13. The assembly of claim 12wherein said hub further includes an annular web section disposedbetween said rim section and said bearing section, said web sectionhaving an axial width less than the axial widths of said rim section andsaid bearing section.
 14. The assembly of claim 8 wherein said bearingsection includes an inner bearing seat defined by a counter-bore. 15.The assembly of claim 14 further including a bearing disposed insidesaid counter-bore of said hub.
 16. The assembly of claim 15 wherein saidbearing comprises a roller bearing having a plurality of roller elementstrapped between inner and outer races.
 17. The assembly of claim 15further including a bushing having a cylindrical outer surface disposedinside said bearing, said bushing having a central through-holeextending along said central axis, an axle shaft disposed in saidthrough hole.